Protein expression systems are used widely to produce desired proteins in biotechnology and more recently to produce sets (combinatorial series) of proteins that are screened for drug discovery purposes. Even though commonly used protein expression systems include those derived from bacteria, yeast, baculovirus/insect and mammalian cells etc., E. coli remains a preferred choice for researchers expressing therapeutic proteins and peptides. There are potential limitations in gene expression in E. coli, however, including toxicity of the foreign peptide, protein insolubility, protease degradation of smaller peptides, improper folding, and inappropriate posttranslational modifications.
The expression of small peptides such as recombinant human parathyroid hormone (rhPTH) in E. coli has remained challenging because of low expression levels. Recombinant fusion expression systems, such as glutathione-S-transferase (GST), thioredoxin (Trx), Sumo, Nus, Intein, and chitin are available for production of small peptides/proteins in the range of 30 to 100 amino acids. Proteins expressed using these available expression systems may be soluble, but the final yields of the protein are often compromised due to the larger size of the fusion partner.
Further regarding parathyroid hormone (PTH, parathormone or parathyrin) is an 84-amino acid polypeptide secreted by the parathyroid gland. This hormone is secreted from cells of the parathyroid glands and finds its major target cells in bone and kidney. Like most other protein hormones, parathyroid hormone is synthesized as a preprophormone. After intracellular processing, the mature hormone is packaged within the Golgi into secretory vesicles, the secreted into blood by exocytosis. It acts to increase the concentration of calcium in the blood by acting upon the parathyroid hormone 1 receptor and the parathyroid hormone 2 receptor.
Recombinant Human Parathyroid hormone (e.g., rhPTH[1-84], or the smaller rhPTH[1-34] or rhPTH[1-31] peptides) is indicated therapeutically for use in postmenopausal women with osteoporosis at a high risk for fracture or with a history of osteoporotic fracture, patients with multiple risk factors for fracture, and for patients who have failed or are intolerant to other available osteoporosis therapy. Additionally, rhPTH is indicated to increase bone mass in men with primary or hypogonadal osteoporosis at high risk of fracture, patients with multiple risk factors for fracture, and for patients who have failed or are intolerant to other available osteoporosis therapy. rhPTH is also indicated for the treatment of men and women with osteoporosis associated with sustained systemic glucocorticoid therapy. Thus, there are important medical needs for improved approaches in producing rhPTH.